1. Field of the Invention
The invention relates to a process for positioning of one mask relative to another mask or positioning of a plurality of masks relative to a workpiece, and to a device for executing the process in an exposure device which is used for production of a semiconductor device, a printed board, a liquid crystal display and the like.
2. Description of Related Art
Production of electrical and electronic components and parts of different types in which processing of structures in the micron range is necessary comprises an exposure process. These electronic parts are semiconductor components, liquid crystal displays, printer heads of the ink jet type, multichip modules in which a plurality of various electronic components are produced on a substrate and thus a module is formed, and the like.
A double-sided exposure system is used as one of these exposure systems, in which mask patterns, which are located above and below the workpiece, are transferred to both of its sides. In such double-sided exposure systems, the masks located above and below are positioned relative to one another, and thus, the patterns which are transferred to the surface and back of the workpiece are positioned relative to one another. Then, the masks are positioned relative to the workpiece. Patterns to be subsequently transferred are positioned exactly relative to the patterns on the workpiece which were formed beforehand.
The above described positioning is conventionally done such that alignment marks of the two masks come to lie on top of one another, and that the alignment marks of the masks and the workpiece come to lie on top of one another.
FIG. 6 schematically shows the conventional positioning of the masks relative to one another and of the masks relative to the workpiece in the above described double-sided exposure system.
In the drawing, the arrangement of a top mask M1, a bottom mask M2, top and bottom projection lenses L1 and L2, and alignment units A1, A2, each of which comprise, for example, a mirror, two lenses and an imager (CCD), is shown. Furthermore, a dummy D for positioning of upper mask M1 relative to lower mask M2 is provided having dummy alignment marks DAM on both side thereof.
In the following, using FIG. 6, the positioning of the masks relative to one another and the positioning of the masks relative to the workpiece are described:
A. Positioning of the masks relative to one another
(1) The dummy D is inserted between the projection lens L1 and the projection lens L2. On both sides of the dummy D alignment marks DAM are recorded. PA1 (2) By irradiation with light of exposure wavelengths, the alignment marks MA1 of upper mask M1 and alignment marks MA2 of lower mask M2 are projected onto the respective sides of the dummy. PA1 (3) Alignment unit A1 is inserted between projection lens L1 and dummy D and Alignment unit A2 is inserted between projection lens L2 and dummy D. PA1 (4) Upper mask M1 and lower mask M2 are moved by a mask carrier movement device (not shown in this figure), such that the projected images of the alignment marks MA1 of upper mask M1, the projected images of the alignment marks MA2 of the lower mask M2 and the dummy alignment marks DAM are brought to lie on top of one another by monitoring of the projected images on dummy D by means of the imagers of alignment units A1 and A2. In this way positioning is performed. PA1 (1) Dummy D and alignment unit A2 are removed and the workpiece is inserted in place of the dummy D. Workpiece alignment marks for positioning relative to the alignment marks of the above described masks are recorded on the workpiece. PA1 (2) Light with nonexposure wavelengths is radiated through the upper mask M1 and projection lens L1. PA1 (3) As a result of the optical construction of projection lens L1 (likewise projection lens L2) with regard to the exposure light wavelengths, an imaging error or aberration occurs, by which deviations of the imaging positions of the alignment marks MA1 of mask M1 from the positions on the top of the workpiece occur. Upper mask M1 and projection lens L1 are, therefore, moved in the direction of the Z-axis (up and down in FIG. 6) at the same time by means of a Z-axis movement device which is not shown in the drawings, and thus the above described deviations are corrected (for this principle, reference is made to Japanese patent application HEI 6-242532 and its counterpart, U.S. Ser. No. 08/540,390, which was filed in the name of one of the present inventors, and which is commonly owned with this application). PA1 (4) The workpiece is moved such that the workpiece alignment marks on the top of the workpiece agree with the projection images of alignment marks MA1 of upper mask M1 by monitoring by means of the imagers of alignment unit A1. In this way, alignment is performed. PA1 (5) Alignment unit A1 are removed. Upper mask M1 which was moved according to above described item (3) and projection lens L1 are moved back into their previous positions. PA1 (1) It is difficult to produce dummy D in which alignment marks DAM of the two sides are in a set positional relationship with respect to one another. PA1 (2) The workpiece does not always have a constant thickness, and can have different thicknesses. It is, therefore, necessary to make available a dummy D for each thickness of the workpiece. Furthermore, there are workpieces in which the positions of the alignment marks are different, the dummy must be prepared according to the workpiece alignment marks. PA1 (3) It is necessary to position the upper mask relative to the lower mask every two to five minutes, and to insert the dummy each time in order to correct deviations of the positional relationship of the upper mask to the lower mask which occur due to temperature changes. Especially when the device is started, there are large temperature changes within it. In this case, it is necessary to frequently correct the above described positional deviations. PA1 (2) The light with nonexposure wavelengths must be monochrome light. This means that width .DELTA.Z in the imaging positions (aberration becomes greater), and as a result thereof, a deviation occurs if the light with nonexposure wavelengths has a wavelength width of .DELTA..lambda. to a certain extent, as is shown in FIG. 8. It is therefore necessary to use monochrome light for the light with nonexposure wavelengths. The wavelength width as monochrome light is a half width of roughly .+-.2 nm. PA1 (3) If, hypothetically, the light with exposure wavelengths is emitted as alignment light, it is necessary to separately place an irradiation unit for the light with exposure wavelengths (inserting it above the upper mask M1) for purposes of partial illumination and to irradiate the alignment marks of upper mask M1. However, even if the irradiation unit for light with exposure wavelengths is used for purposes of partial illumination, in the areas used for positioning on the workpiece, a circuit pattern or the like cannot be formed since, during positioning, the resist in the vicinity of the workpiece alignment marks is exposed to the action of exposure light. As a result, the areas which can be used become smaller. PA1 (4) In the case of using light with exposure wavelengths, it is difficult for the light to reach the surface of the workpiece (or the surface of the base), since the light with exposure wavelengths is easily absorbed by the photoresist. Therefore, it becomes difficult to determine the alignment marks.
B. Positioning of the masks relative to the workpiece
In the above described art, however, the following is considered disadvantageous:
A. Positioning of the upper mask relative to the lower mask
Especially in the case of alignment using dummy D with a thickness which differs from the thickness of the workpiece, the two sides of the workpiece cannot be exposed at the same time, and step-by-step exposure must be performed. This means that, here, it is necessary to move the positions of the projection lens and the workpiece in the direction of the Z-axis, and to expose the other side of the workpiece after alignment and exposure have been performed for the first side of the workpiece.
B. Positioning of the upper and lower masks relative to the workpiece and in double-sided exposure
(1) Due to use of light with nonexposure wavelengths during workpiece alignment, the aberration correction described in item (3) in the above described prior art must be done for the respective alignment.
However, it is unnecessary to accomplish the above described aberration correction if, with respect to the light with the exposure wavelengths (for example of the g-line), high image resolution is accomplished and a projection lens with a small aberration both with respect to light with exposure wavelengths and also with respect to light with nonexposure wavelengths (for example of the e-line) is used, as is shown in FIG. 7. However it is difficult to produce a lens of this type. PA2 In the case of using monochrome light however interference of light which is reflected from the photoresist surface occurs with the light which is reflected from the workpiece surface (or the surface of the base). This means that the resist layer thickness is roughly 1 to 5 microns; however, it has a variation of roughly 0.1 micron to 0.2 microns, as is shown in FIG. 9. Therefore as a result of the variation of the resist layer thickness, interference bands occur, as is shown for example in FIG. 10; this makes alignment difficult.